The invention relates to processes for manufacturing thin components made of a zirconium-based alloy which are intended to be used in water-cooled nuclear reactors and especially, among these, in pressurized-water reactors.
One particularly important application of the invention is in the manufacture of sheets intended to be cut so as to form structural components of the backbone of a nuclear fuel assembly and especially to form spacer grid straps for the fuel rods.
Such structural components are required to have, simultaneously, good resistance to the coolant, consisting of a high-temperature aqueous medium, low free growth in the direction of the large dimension of the component, and reduced creep in the case of components subjected to stresses. In addition, the manufacturing process must not result in a high scrap rate.
The aim of the invention is especially to provide a process allowing these objectives to be achieved, especially by avoiding the presence of the xcex2Zr phase which has an unfavorable effect from the oxidation standpoint. The aim of the invention is also, secondarily, to provide a process employing a zirconium-based alloy that can also be used to form cladding tubes, which are in contact not only with the aqueous medium but also with the fuel.
Tubes have already been proposed (EP-A-0 720 177) which are made of a zirconium-based alloy also containing 50 to 250 ppm iron, 0.8 to 1.3 wt % niobium, less than 1600 ppm oxygen, less than 200 ppm carbon and less than 120 ppm silicon. Such an alloy is drawn and then cold-rolled in at least four passes (the term xe2x80x9crollingxe2x80x9d denoting, in the case of tube, passage over a mandrel against which the blank is pressed by forming rolls), in several passes, with intermediate heat treatments between 560xc2x0 C. and 620xc2x0 C.
At these relatively low temperatures, advantageous from the standpoint of corrosion resistance, recrystallization of the alloy, necessary for the next rolling pass to be carried out properly, requires a long time.
A process according to the invention makes it possible to produce flat thin structural components by a process that can be employed on a continuous line. According to this process, a blank made of a zirconium-based alloy is formed, said alloy also containing, by weight, beside the inevitable impurities, 0.8 to 1.3% niobium, 500 to 2000 ppm oxygen and 5 to 35 ppm sulfur and, optionally, Fe, Cr and V with a total content of less than 0.25% and tin with a content of less than 300 ppm. A blank, resulting from a xcex2 hardening operation and a hot-rolling pass, is rolled in at least three cold-rolling passes with intermediate annealing heat treatments, one of these intermediate heat treatments or a prior heat treatment before the first cold-rolling pass being carried out for a long time, of at least 2 hours, at a temperature below 600xc2x0 C., and all the optional heat treatments which follow the long treatment being carried out at a temperature generally between 610 and 620xc2x0 C. for at most 15 min, in general 2 to 10 min.
Often 1100 to 1800 ppm oxygen and 10 to 35 ppm sulfur will be used.
One advantage of xe2x80x9cshortxe2x80x9d treatments of at most 15 min is that they can be carried out in continuous furnaces. However, they may be at a temperature causing the xcex2Zr phase to appear, since this phase will be eliminated by the xe2x80x9clongxe2x80x9d treatment.
In a first method of implementation, the number of cold-rolling passes is only three. The first intermediate heat treatment is at a temperature exceeding 620xc2x0 C., causing the xcex2Zr phase to appear, and of short duration, compatible with the use of continuous furnaces. On the other hand, the treatment which immediately precedes the final rolling is at a low temperature, below 600xc2x0 C., and of a duration exceeding 2 hours, which can be carried out in a bell furnace. This operation eliminates practically all the xcex2Zr phase. A treatment below 560xc2x0 C. is especially possible, with a duration which then exceeds 5 hours.
In a second method of implementation, four cold-rolling passes are carried out with short intermediate annealing operations between the first two or first three passes at a temperature resulting in the appearance of the xcex2Zr phase. The xcex2Zr phase is then eliminated by a long annealing operation (more than 2 hours) at a temperature below 600xc2x0 C. before the final or penultimate rolling, depending on the case. An annealing operation of more than 5 hours below 560xc2x0 C. allows a similar result to be achieved.
In yet another case, the process comprises four (or more) cold-rolling passes and the treatment of long duration at a temperature below 600xc2x0 C. (often below 560xc2x0 C.) is carried out immediately after hot rolling. All the subsequent annealing operations are below 620xc2x0 C., short (less than 15 min) and carried out in a continuous furnace.
In all cases, a final recrystallization annealing operation is carried out at a temperature low enough to prevent the xcex2Zr phase from appearing, that is to say below 620xc2x0 C.
The thin component thus obtained constitutes a sheet which no longer undergoes thermometallurgical treatment before use, but simply planishing, pickling, inspection and, finally, cutting operations.
The manufacture may be carried out with continuous annealing furnaces for all the short heat treatments at high temperature. The annealing operation to eliminate the xcex2Zr phase requires only a time of several hours, typically 5 to 15 h, between 520 and 580xc2x0 C., for example in a bell furnace.
All of the heat treatments are carried out in an inert atmosphere or under vacuum.
The presence of sulfur with a low content improves the hot creep in an aqueous medium. An oxygen content of between 1000 and 1600 ppm is advantageous. It may be adjusted by a deliberate and controlled addition of zirconium.
The alloy used also lends itself to the formation of claddings, by a process involving several rolling passes in a pilger rolling mill, this time with long intermediate heat treatments at a temperature low enough not to make the xcex2Zr phase appear.
The same alloy containing 5 to 35 ppm (especially 10 to 35 ppm) sulfur consequently makes it possible to form, from ingots of the same composition, both flat components cut from a sheet and cladding tubes or nuclear fuel assembly guide tubes.